Summary of GSM RF Power control

[Harald Welte]

On Thu, Jun 18, 2009 at 11:58:48PM +0200, Harald Welte wrote:
> On Thu, Jun 18, 2009 at 11:32:55PM +0200, Harald Welte wrote:
> > If we use the knowledge of the behavior as described above, we can also deduct:
> > 
> > * the BS-11 is configured to a NM_ATT_RF_MAXPOWR_R of 0, i.e. it will transmit
> >   with the power level that is configured by LMT / ipaccess_config.  By default
> >   this is set to 30mW 
> > 
> > * the nanoBTS 900 has 20dBm (1800 is 23dBm) TRX output power.  bsc_hack is 
> >   configured to a NM_ATT_RF_MAXPOWR_R of 12, i.e. 24dB.  This means we are
> >   transmitting with a mere -4dBm (398uW) or -1dBm (794uW) which would be _really_
> >   low.  So either the nanoBTS are not following specs, or we're really
> >   transmitting something that would barely be possible to receive.  Or my
> >   calculations are wrong ;)
> 
> I've now also realised that every CHANNEL ACTIVATE message contains a BS power
> and MS power IE with 0x0f as value, i.e. another 30dB decrease in initial power
> levels.  This really makes me suspicious...  at levels that low, everything
> should never be working.  Probably the MS and BTS cannot operate below a
> certain level, and thus they choose whatever is the minimum level they support.

For the BTS I'm still puzzled.  For the MS, a power level of 0xf (15) means:
13dBm (19mW) output power in GSM900 and 0dBm (1mW) in GSM1800 - which would
probably work well unless there's actual uplink interference on the respective
channel.

-- 
- Harald Welte <laforge at gnumonks.org>           http://laforge.gnumonks.org/
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